You may already do this but letting the egg whites come to room temperature makes a difference. Also, I find that young eggs fluff up more than older eggs, but that's just an observation. (The image comes from www.knivescookslove.com)

I think it may have something to do with humidity--i think a drier environment makes for fluffier meringue. I have no science to back this up; it is just my observation. Also, I personally don't like cream of tartar in meringue (i can always taste it.) but a pinch of salt really helps the egg whites beat up light and fluffy.